Hydraulic control system for operating the blade of a bulldozer

ABSTRACT

A hydraulic control system for operating the blade of a bulldozer which has a fluid pump, a fluid tank, a control valve for controlling the communication of the fluid fed from the fluid pump back to the fluid tank with three ports, a tilt-locking valve for communicating the fluid fed from the control valve with control cylinders and for shutting off the communication with the three ports, check valves for selectively passing only the fluid fed to the control cylinder side, throttle valves for passing the fluid at a constant rate of amount regardless of the hydraulic pressure of the fluid from the control cylinder side to the fluid tank side, and a manual control lever mechanism for operating the control valve and the tilt-locking valve. Thus, the angle of the edge of the blade of the bulldozer may be accurately varied, and the tilting of the blade is also conducted with great ease.

United States Patent 1 Jindai HYDRAULIC CONTROL SYSTEM FOR OPERATING THE BLADE OF A BULLDOZER Hideo Jindai, Osaka, Japan Kabushiki Kaisha Komatsu Seisakusho, Tokyo, Japan Filed: Feb. 29, 1972 Appl. No.: 230,359

Inventor:

Assignee:

US. Cl. 172/803, 91/413 Int. Cl E021 3/76 Field of Search 172/801-809; 91/413 References Cited UNITED STATES PATENTS Primary Examiner-Robert E. Pulfrey Assistant Examiner-Stephen C. Pellegrino Attorney-James E. Armstrong et al.

[5 7 ABSTRACT A hydraulic control system for operating the blade of a bulldozer which has a fluid pump, a fluid tank, a control valve for controlling the communication of the fluid fed from the fluid pump back to the fluid tank with three ports, a tilt-locking valve for communicating the fluid fed from the control valve with control cylinders and for shutting off the communication with the three ports, check valves for selectively passing only the fluid fed to the control cylinder side, throttle valves for passing the fluid at a constant rate of amount regardless of the hydraulic pressure of the fluid from the control cylinder side to the fluid tank side, and a manual control lever mechanism for operating the control valve and the tilt-locking valve. Thus, the angle of the edge of the blade of the bulldozer may be accurately varied, and the tilting of the blade is also conducted with great case.

3 Claims, 3 Drawing Figures 2 Sheets-S aet 1 Patented July 31, 1973 3,749,181

2 Sheets-Sheet f:

HYDRAULIC CONTROL SYSTEM FOR OPERATING THE BLADE OF A BULLDOZER CROSS-REFERENCE TO RELATED APPLICA- TlONS The prior art of the present invention is disclosed in U.S. Pat. No. 3,184,869.

BACKGROUND OF THE INVENTION This invention relates to a control device for the blade of a bulldozer, and more particularly to a hydraulic control system for operating the blade of a bulldozer.

Prior to the present invention, as disclosed in U.S. Pat. No. 3,184,869, when the blade of the dozer is tilted, one jack is extended, while the other is contracted. Therefore, even if the oil flow rates from both jacks are the same upon tilting of the dozer blade, the strokes of both jacks are different from each other due to the differences of the sectional areas of the jacks at the head side and the bottom side. Thus the angle of the edge of the blade is varied simultaneously upon tilting of the blade. Since both the jacks are moved, when the blade is tilted back or is dumped to a maximum extent, it cannot be tilted to both sides.

Further, the stroke of the jack required for tilting the blade is generally to be shorter than that required for pitching it so that its amount is approximately below one-third. It is, therefore, necessary to define the stroke upon tilting of the blade. However, the hydraulic control system disclosed in the above U.S. Patent does not disclose this point at all. In addition, when the blade is returned from tilting to the horizontal position, the operator must judge the position and location by sight due to the blade structure.

SUMMARY OF THE INVENTION This invention contemplates to eliminate the aforementioned disadvantages of the conventional hydraulic control system for a dozer, and to provide a novel and improved hydraulic control system for operating the blade of a bulldozer.

It is, therefore, an object of the present invention to provide a hydraulic control system for operating the blade of a bulldozer which may positively vary the angle of the edge of the blade thereof.

It is another object of the present invention to provide a hydraulic control system for operating the blade of a bulldozer which may easily tilt the blade thereof.

It is a further object of the present invention to provide a hydraulic control system for operating the blade of a bulldozer which may dump, tilt back or tilt the blade from the cab of the bulldozer.

It is still another object of the present invention to provide a hydraulic control system for operating the blade of a bulldozer which may tilt the blade by changing the stroke of only one cylinder without varying the angle of the edge of the blade.

It is still another object of the present invention to provide a hydraulic control system for operating the blade of a bulldozer which may tilt in one direction enabling movement of the tilt cylinder even if the blade is dumped or tilted back to a maximum extent.

Briefly, in accordance with the present invention, the foregoing and other objects are attained, in one aspect, by the provision of a hydraulic control system for operating the blade of a bulldozer which comprises a control valve having a spool including a first port for communicating a fluid port connected to a fluid pump with a first fluid port and the fluid port connected to a fluid tank with second fluid port, a second port for shutting off these communication, and a third port for communication the fluid port connected to the fluid tank with the second fluid portand the fluid port connected to the fluid tank with the first fluid port, a tilt-locking valve having a spool including first and third ports for communicating third fluid port with the bottom side of one control cylinder with a piston and a fourth fluid port with the head side of the one control cylinder with the piston, and a second port for shutting off these communications and for communicating a third fluid port with the head side of the other control cylinder with a piston and a fourth fluid port with the bottom side of the other control cylinder with the piston, first and second pairs of check valves for passing only the fluid fed to the control cylinder side, a first pair of conduits connected from the first fluid port of said control valve to the bottom side of said other control cylinder with the piston and connected from the first fluid port to the third fluid port of said tilt-locking valve through said first pair of check valves, respectively, a second pair of conduits connected from the second fluid port to the fourth fluid port and connected from the second ports to the head side of said other control cylinder with the piston through said second pair of check valves, respectively, and a pair of throttle valves connected from the control cylinder side to the fluid tank side for passing the fluid at a constant rate of amount regardless of the hydraulic pressure of the fluid from the control cylinder side to the fluid tank side.

According to the present invention, there is, in another aspect, provided a hydraulic control system for operating the blade of a bulldozer which comprises, in addition to the above components, a manual control lever mechanism connected to the spools of said control valve and said tilt-locking valve for operating said control valve and said tilt-locking valve, and a mechanism connected to said manual control lever mechanism for shutting off the connection of the spools of said control valve and said tilt-locking valve when the blade is tilted leftwardly or rightwardly.

BRIEF DESCRIPTION OF THE DRAWINGS A more complete appreciation of the present inven tion will be readily obtained as the same becomes better understood by reference to the following detailed description when taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a hydraulic circuit diagram of the hydraulic control system for operating the blade of a bulldozer according to one embodiment of the present invention;

FIG. 2 is a sectional view of the valve structure of the hydraulic control system of the present invention; and

FIG. 3 is a partial schematic side view of the bulldozer in which the present invention is incorporated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference is now made to the drawings, wherein like reference numerals designate identical, or corresponding parts throughout the several views, and more particularly to FIGS. 1 and 2, wherein the hydraulic control system for operating the blade of a bulldozer of the present invention is shown as one embodiment thereof.

Reference numeral 1 illustrates a brace or control valve which has a fluid port 3 connected to a fluid tank 2, a fluid port 4 connected to a fluid pump PF, a fluid port 5 connected to the fluid pump PF for passing the fluid only from the fluid pump PF to the control valve 1 side through a check valve, an opposite port I disposed opposite to the port 3 connected to the fluid tank 2, and an opposite port H disposed opposite to the port 5 connected to the fluid pump PF at a sleeve side. The control valve 1 also has a first port A for communicating the fluid port 5 connected to the fluid pump PF with the opposite port I and the fluid port 3 with the opposite port H, a second port B for communicating the fluid port 4 with the fluid port 3, and a third port C for communicating the fluid port 3 connected to the fluid tank 2 with the opposite port I and the fluid port 5 with the opposite port H at the side of a spool. The opposite port I communicates through a fluid passage 9 with fluid ports LB2 and RB2 of throttle valve group 10 of the left side in the drawing, which group 10 has throttle valves a, b, e and ffor passing the fluid from the bottom sides of the right and left brace or control cylinders 40 and 13 to the tank 2 side at a constant rate of amount regardless of the hydraulic pressure of the fluid therein. The fluid port 11 communicated with a fluid port LB2 is connected through fluid passage 12 to the bottom side of the right brace or control cylinder 13. The fluid port 14 communicated with a fluid port RB2 is connected through a fluid passage 17 to a fluid port 16 of a tilt-locking valve 15. The fluid passage 9 is communicated through fluid passage 18 with the fluid passage 12, and a check valve LB] for passing the fluid only from the fluid passage 9 to the fluid passage 12 through the fluid passage 18 is provided. The fluid passage 9 is connected through a fluid passage 19 to a fluid port 37 of the brace side of the tilt-locking valve 15, and a check valve RBI for passing the fluid only from the oil passage 9 to the fluid passage 19 is provided. The opposite port H of the control valve 1 is communicated through a fluid passage 20 with the fluid ports RH2 and LH2 of throttle valve group 21 of right side in the drawing, which group 21 has throttle valves c, d, g and h for passing the fluid from the head sides of the right and left brace or control cylinders 40 and 13 to the tank 2 at a constant rate of amount regardless of the hydraulic pressure of the fluid therein. The fluid port 22 communicated with the fluid port R32 is connected through a fluid passage 23 to the fluid port 24 of the brace side of the tilt-locking valve 15, and the fluid port 25 communicated with the fluid port LH2 is connected through a fluid port 26 to the head side LH of the brace or control cylinder 13 of the right side. The fluid passage 20 is connected through a fluid passage 27 to the fluid port 38 of brace side of the tilt-locking valve 15, and a check valve RH! for passing the fluid only from the fluid passage 20 to the fluid passage 27 is provided. The fluid passage 20 is communicated through a fluid passage 28 with the fluid passage 26, and a check valve LHI for passing the fluid from the fluid passage 20 to the fluid passage 26 through the fluid passage 28 is provided. A fluid port 29 provided at the sleeve side of the tiltlocking valve is connected to the fluid passage 12 through a fluid passage 30, and a fluid port 31 is connected through a fluid passage 32 to the fluid passage 26. Further, the port 33 disposed opposite to the fluid port 16 is connected to the bottom side RB of the control cylinder 40 of left side, and a fluid port 35 disposed opposite to the fluid port 24 is connected to the head side RH of the control cylinder 40. The tiltlocking valve 15 has first and third ports A and C for communicating the fluid ports 16 and 37 with a fluid port 33 and fluid ports 24 and 38 with a fluid port 34, respectively, and a second port B for communicating the fluid port 16 with the fluid port 29 and fluid port 24 with the fluid port 31 at the side of a spool. The spools of the control valve 1 and the tilt-locking valve 15 are interlocked by one manual control lever 35. This interlocking thereof is transferred by a knob or transfer control lever 36. The tilt-locking valve 15 is integrally constructed with the right and left check valve groups 10 and 21. The piston rods 13a and 34a of the control cylinders 13 and 40, respectively are connected to the upper both sides of the blade 39 of the bulldozer (See FIG. 3).

In operation of the thus constructed hydraulic control system, when the blade 39 is dumped, the manual control lever 35 is placed in a forward position so that the spool of the control valve 1 is placed in the position P at the port A thereof at the same time the spool of the tilt-locking valve 15 is placed to the position P at the port A with the result that the fluid fed from the pump PF is directed from the fluid port 5 of the control valve 1 through the port I, fluid passages 9, l8 and 12 through the check valve LBl to the bottom side LB of the right control cylinder 13, and the fluid is also directed from the fluid passage 9 through the fluid passage 19 through the check valve RBI to the fluid port 37 of the tilt-locking valve 15 or through the fluid passage 17 to the fluid port 16 of the tilt-locking valve 15 through the fluid port 33 of the tilt-locking valve 15 to the bottom side RB of the left control cylinder 40. In this case, the fluid of the head side LH of the right control cylinder 13 is directed through the fluid passage 26, fluid ports 25 and LH2 of the right throttle valve group 21, fluid passage 20, fluid port H of the control valve 1 and fluid port 3 to the fluid tank 2. And, the fluid of the head side RH of the left control cylinder 40 is directed through the fluid ports 34 and 24 of the tiltlocking valve 15, fluid passage 23, fluid ports 22 and RH2 of the throttle valve group 21, fluid passage 20, fluid ports H and 3 of the control valve 1 to the tank 2.

When the blade 39 is tilted back, the manual control lever 35 is placed in a rearward position so that the spool of the control valve 1 is placed to the position P at the port B thereof at the same time the spool of the tilt-locking valve 15 is placed to the position P' at the port C thereof with the result that the fluid fed from the pump PF is directed from the port 5 of the control valve 1 through the port H, fluid passages 20 and 27 or 23 through the check valve RHl, fluid ports 38 or 24 and 34 of the tilt-locking valve 15 to the head side RH of the left control cylinder 40, and the fluid is also directed from the fluid passage 20 through the fluid passage 28 through the check valve LHl and fluid passage 26 to the head side LB of the right control cylinder 13 or through the fluid ports LH2 and 25 through the fluid passage 26 to the head side LB of the control cylinder 13. In this case, the fluid of the bottom side LB of the right control cylinder 13 is directed through the fluid passage 12, fluid ports 11 and LB2 of the left throttle valve group 10, fluid passage 9, fluid port I of the control valve 1 and fluit port 3 to the fluid tank 2. And the fluid of the bottom side RB of the control cylinder 40 is directed through the fluid ports 33 and 16 of the tilt-locking valve 15, fluid passage 17, fluid ports 14 and RB2 of the left throttle valve group 10, fluid passage fluid ports I and 3 of the control valve 1 to the tank When the blade 39 is tilted rightwardly, the knob 36 is pushed in so as to shut off the interlocking of the control valve 1 and the tilt-locking valve and the manual control lever 35 is placed forwardly so that the spool of the control valve 1 is placed to the position P at the port A thereof at the same time the spool of the tiltlocking valve 15 is placed to the position P at the port B thereof with the result that the fluid fed from the pump PF is directed from the port 5 of the control valve 1 through the port I, fluid passages 9, 18 and 12 through the check valve L131 to the bottom side LB of the right control cylinder 13, and the fluid is also directed from the fluid passage 9 through the fluid ports R82 and 14 of the throttle valve group 111, fluid passage 17, fluid ports 16 and 29, fluid passage 30 to the bottom side LB of the right control cylinder 13. In this case, the fluid of the head side LH of the control cylinder 13 is directed through the fluid passage 26, fluid ports 25 and LHZ of the throttle valve group 21, fluid passage 20, fluid ports H and 3 of the control valve 1 to the tank 2. Further, the fluid of the head side LH of the control cylinder 13 is also directed through the fluid passage 32, fluid ports 31 and 24 of the tiltlocking valve 15, fluid passage 23, fluid ports 22 and RHZ of the throttle valve group 21, fluid passage 20, fluid ports H and 3 of the control valve 1 to the tank 2.

When the blade 39 is tilted in a leftward direction, the knob 36 is pushed in so as to shut off the interlocking of the control valve 1 and the tilt-locking valve 15 and the manual control lever 35 is placed in a rearward position so that the spool of the control valve 1 is placed to the position P at the port C thereof and the spool of the tilt-locking valve 15 is maintained as before with the result that the fluid fed from the pump PF is directed from the port 5 of the control valve 1 through the port H, fluid passages 20, 28 and 26 through the check valve L111 to the head side LH of the right control cylinder 13, and the fluid of the bottom side LB of the control cylinder 13 is directed from the fluid passage 12 through the fluid ports 11 and U32 of the throttle valve group 141, fluid passage 9, fluid ports I and 3 of the control valve 1 to the tank 2. The fluid of the bottom side LB of the control cylinder 13 is directed from the fluid passage 30 through the fluid ports 29 and 16 of the tilt-locking valve 15, fluid passage 17, fluid ports 14 and R132 of the throttle valve group 10, fluid passage 9, fluid ports 1 and 3 of the control valve 1 to the tank 2. In this case, the fluid fed from the fluid passage 20 to the fluid ports R112 and 22 of the throttle valve group 21 is directed from the fluid passage 23 through the fluid passage 32 to the fluid passage 26.

It should be understood from the foregoing description that since the hydraulic control system of the present invention is thus constructed, by placing the first port of the control valve in a communicating position and simultaneously providing the first port of the tiltlocking valve in a communicating position, hydraulic fluid is applied to the bottom sides of both the control cylinders so as to dump the blade. By placing the third port of the control valve in a communicating position and providing the third port of the tilt-locking valve to the communicating position, hydraulic fluid is applied to the bottom sides of both the control cylinders so that the fluid of the bottom side thereof is directed to the tank so as to tilt back the blade. By placing the first port of the control valve in a communicating position and providing the second port of the tilt-locking valve in a communicating position, hydraulic fluid is applied to the bottom side of one control cylinder so that the fluid of the head side of the control cylinder is directed to the tank so as to tilt the blade to one direction, and by placing the third port of the control valve to the communicating position and providing the second port of the tilt-locking valve to the communicating position,hydraulic fluid is applied to the head side of the one control cylinder so that the fluid of the bottom side of the control cylinder is directed to the tank so as to tilt the blade to the other direction. Thus, the angle of the edge of the blade may be positively varied by the hydraulic control system of the present invention.

it should also be understood that since the hydraulic control system is so constructed as to comprise a manual control lever mechanism connected to the spools of the control valve and the tilt-locking valve for operating the control valve and the tilt-locking valve and a mechanism connected to the manual control lever mechanism for shutting off the connection of the spools of the control valve and the tilt-locking valve when the blade is titlted leftwardly or rightwardly, the blade may be dumped and tilted back by the interlocking operation of the spools of both the control valve and the tiltlocking valve, and by providing the manual control lever in the cab of the bulldozer, these operations may be conducted from the cab.

Other variations of the foregoing embodiments of the invention will be apparent to those skilled in the art.

What is claimed is:

1. A hydraulic control system for operating the blade of a bulldozer comprising a control valve having a spool and including a first port for communicating a fluid port connected to fluid pump with a first fluid port with the fluid port connected to a fluid tank and with a second fluid port,

a second port for shutting off these communications,

and

a third port for communicating the fluid port connected to the fluid tank with the second fluid port and the fluid port connected to the fluid tank with the first fluid port,

a tilt-locking valve having a spool and including first and third ports for communicating a third fluid port with the bottom side of one control cylinder with a piston and a fourth fluid port connecting with a head side of the one control cylinder with the piston, and a second port for shutting off these communications and for communicating the third fluid port with the head side of the other control cylinder with a piston and the fourth fluid port with the bottom side of the control cylinder and the piston,

first and second pairs of check valves for selectively passing only the fluid fed to the control cylinder side,

a first pair of conduits connected with the first fluid port of said control valve and to the bottom side of said other control cylinder with the piston and,

connected with the first fluid port to the third fluid port of said tilt-locking valve through said first pair of check valves, respectively,

a second pair of conduits connected from the second fluid port to the fourth fluid port and connected from the seond ports to the head side of said other control cylinder, and with the piston through said second pair of check valves, respectively, and,

a pair of throttle valves connected from the control cylinder side to the fluid tank side for passing the Y fluid at a constant amount rate regardless of the hydraulic pressure of the fluid from the control cylinder side to the fluid tank side.

2. A hydraulic control system as set forth in claim 1, further comprising a manual control lever mechanism connected to the spools of said control valve and said tilt-locking valve for operating said control valve and said tilt-locking valve, and a mechanism connected to said manual control lever mechanism for shutting off the connection of the spools of said control valve and said tilt-locking valve when the blade is tilted in a leftward or rightward direction, respectively.

3. A hydraulic control system for operating the blade of a bulldozer having a fluid pump for feeding hydraulic fluid, and a fluid tank, comprising a control valve having a first fluid port connected to said tank, a second fluid port connected to said fluid pump, a third fluid port connected to said fluid pump for passing selectively the fluid only from said fluid pump to said control valve side through a check valve,

a first opposite port disposed opposite to the first port connected to said fluid tank, and a second opposite port disposed opposite to the third port connected to said fluid pump for selectively passing the fluid only from said fluid pump to said control valve side through the check valve at a sleeve side and,

also having a first port for communicating the second fluid port connected to said fluid pump with its opposite port and the first fluid port connected to said fluid tank with its opposite port,

a second port for shutting off these communications and,

a third port for communicating the first fluid port connected to the fluid tank with the second port and the first fluid port connected to the fluid tank with the first port at a spool side,

a pair of control cylinders each with a piston each connected to either upper side of said blade,

first and second pairs of check valves for selectively passing only the fluid fed to said control cylinder side and connected to the first pair through conduits to the opposite port of said control valve disposed opposite to the first fluid port connected to said fluid tank and connected at the second pair through conduits to the opposite port of said control valve disposed opposite to the third fluid port connected to said fluid pump for selectively passing the fluid only from said fluid pump to said control valve side,

a pair of throttle valve groups for passing the fluid at a constant amount rate regardless of the hydraulic pressure of the fluid from the control cylinder side to the fluid tank side and connected at one couple throttle valves through conduits to the opposite port of said control valve disposed opposite to the first fluid port connected to said fluid tank and connected at the other couple throttle valves through conduits to the opposite port of said control valve disposed opposite to the third fluid port connected to said fluid pump for selectively passing the fluid only from said fluid pump to said control valve side,

a tilt-locking valve having a first fluid port connected to the other side of one of the first pair of check valves through a conduit, a second fluid port connected to the other side of one of the couple throttle valves of the one throttle valve group through a conduit, a first port connected to the other side of one of the couple throttle valves of the other throttle valve group through a conduit, a second port connected to the other side of one of the second pair of check valves through a conduit, an opposite port disposed opposite to the first fluid port connected to the other side of one of the first pair of check valves and connected to the bottom side of one of said control cylinders through a conduit and connected to the other sides of the other of the first pair of check valves and of the other side of the couple of the one throttle valve group,

an opposite port disposed diametrically to the second fluid port connected to the other side of one of the couple throttle valves of the one throttle valve group through a conduit and connected to the bottom side of the other side of said control cylinders through a conduit,

an opposite port disposed to the second fluid port connected to the other side of one of the couple throttle valves of the other throttle valve group through a conduit and connected to the head side of the other of the control cylinders through a conduit and,

an opposite port disposed diametrically to the port connected to the other side of one of the second pair of check valves through a conduit and connected to the head side of the one of the control cylinders through a conduit at a sleeve side and connected to both the other sides of the other of the couple throttle valves of the other throttle valve group and of the other of the second pair of check valves and,

also having first and third ports for communicating the third fluid port with the bottom side of one control cylinder with a piston and a fourth fluid port with the head side of the one control cylinder with the piston and second port for shutting off these communications and for communicating the third fluid port with the head side of the other control cylinder with a piston and the fourth fluid port with the bottom side of the other control cylinder with the piston at a spool side,

a manual control lever mechanism connected to the spools of said control valve and said tilt-locking valve for operating said control valve and said tiltlocking valve and,

a transfer control lever interlocked to said manual control lever mechanism for switching the connection of the manual control lever to either spool of said control valve and said tilt-locking valve.

t II! t i i 

1. A hydraulic control system for operating the blade of a bulldozer comprising a control valve having a spool and including a first port for communicating a fluid port connected to fluid pump with a first fluid port with the fluid port connected to a fluid tank and with a second fluid port, a second port for shutting off these communications, and a third port for communicating the fluid port connected to the fluid tank with the second fluid port and the fluid port connected to the fluid tank with the first fluid port, a tilt-locking valve having a spool and including first and third ports for communicating a third fluid port with the bottom side of one control cylinder with a piston and a fourth fluid port connecting with a head side of the one control cylinder with the piston, and a second port for shutting off these communications and for communicating the third fluid port with the head side of the other control cylinder with a piston and the fourth fluid port with the bottom side of the control cylinder and the piston, first and second pairs of check valves for selectively passing only the fluid fed to the control cylinder side, a first pair of conduits connected with the first fluid port of said control valve and to the bottom side of said other control cylinder with the piston and, connected with the first fluid port to the third fluid port of said tilt-locking valve through said first pair of check valves, respectively, a second pair of conduits connected from the second fluid port to the fourth fluid port and connected from the seond ports to the head side of said other control cylinder, and with the piston through said second pair of check valves, respectively, and, a pair of throttle valves connected from the control cylinder side to the fluid tank side for passing the fluid at a constant amount rate regardless of the hydraulic pressure of the fluid from the control cylinder side to the fluid tank side.
 2. A hydraulic control system as set forth in claim 1, further comprising a manual control lever mechanism connected to the spools of said control valve and said tilt-locking valve for operating said control valve and said tilt-locking valve, and a mechanism connected to said manual control lever mechanism for shutting off the connection of the spools of said control valve and said tilt-locking valve when the blade is tilted in a leftward or rightward direction, respectively.
 3. A hydraulic control system for operating the blade of a bulldozer having a fluid pump for feeding hydraulic fluid, and a fluid tank, comprising a control valve having a first fluid port connected to said tank, a second fluid port connected to said fluid pump, a third fluid port connected to said fluid pump for passing selectively the fluid only from said fluid pump to said control valve side through a check valve, a first opposite port disposed opposite to the first port connected to said fluid tank, and a second oppositE port disposed opposite to the third port connected to said fluid pump for selectively passing the fluid only from said fluid pump to said control valve side through the check valve at a sleeve side and, also having a first port for communicating the second fluid port connected to said fluid pump with its opposite port and the first fluid port connected to said fluid tank with its opposite port, a second port for shutting off these communications and, a third port for communicating the first fluid port connected to the fluid tank with the second port and the first fluid port connected to the fluid tank with the first port at a spool side, a pair of control cylinders each with a piston each connected to either upper side of said blade, first and second pairs of check valves for selectively passing only the fluid fed to said control cylinder side and connected to the first pair through conduits to the opposite port of said control valve disposed opposite to the first fluid port connected to said fluid tank and connected at the second pair through conduits to the opposite port of said control valve disposed opposite to the third fluid port connected to said fluid pump for selectively passing the fluid only from said fluid pump to said control valve side, a pair of throttle valve groups for passing the fluid at a constant amount rate regardless of the hydraulic pressure of the fluid from the control cylinder side to the fluid tank side and connected at one couple throttle valves through conduits to the opposite port of said control valve disposed opposite to the first fluid port connected to said fluid tank and connected at the other couple throttle valves through conduits to the opposite port of said control valve disposed opposite to the third fluid port connected to said fluid pump for selectively passing the fluid only from said fluid pump to said control valve side, a tilt-locking valve having a first fluid port connected to the other side of one of the first pair of check valves through a conduit, a second fluid port connected to the other side of one of the couple throttle valves of the one throttle valve group through a conduit, a first port connected to the other side of one of the couple throttle valves of the other throttle valve group through a conduit, a second port connected to the other side of one of the second pair of check valves through a conduit, an opposite port disposed opposite to the first fluid port connected to the other side of one of the first pair of check valves and connected to the bottom side of one of said control cylinders through a conduit and connected to the other sides of the other of the first pair of check valves and of the other side of the couple of the one throttle valve group, an opposite port disposed diametrically to the second fluid port connected to the other side of one of the couple throttle valves of the one throttle valve group through a conduit and connected to the bottom side of the other side of said control cylinders through a conduit, an opposite port disposed to the second fluid port connected to the other side of one of the couple throttle valves of the other throttle valve group through a conduit and connected to the head side of the other of the control cylinders through a conduit and, an opposite port disposed diametrically to the port connected to the other side of one of the second pair of check valves through a conduit and connected to the head side of the one of the control cylinders through a conduit at a sleeve side and connected to both the other sides of the other of the couple throttle valves of the other throttle valve group and of the other of the second pair of check valves and, also having first and third ports for communicating the third fluid port with the bottom side of one control cylinder with a piston and a fourth fluid port with the head side of the one control cylinder with the piston and second port for shutting off these communications and for comMunicating the third fluid port with the head side of the other control cylinder with a piston and the fourth fluid port with the bottom side of the other control cylinder with the piston at a spool side, a manual control lever mechanism connected to the spools of said control valve and said tilt-locking valve for operating said control valve and said tilt-locking valve and, a transfer control lever interlocked to said manual control lever mechanism for switching the connection of the manual control lever to either spool of said control valve and said tilt-locking valve. 